displacement of the earth in its annual movement round the sun would
produce an apparent shift in the place of the nearer of the two stars
relatively to the other, supposed to be much more remote. If this
shift could be measured, then the distance of the nearer of the stars
could be estimated with some degree of precision.

As has not unfrequently happened in the history of science, an effect
was perceived of a very different nature from that which had been
anticipated. If the relative places of the two stars had been
apparently deranged merely in consequence of the motion of the earth,
then the phenomenon would be an annual one. After the lapse of a
year the two stars would have regained their original relative
positions. This was the effect for which William Herschel was
looking. In certain of the so called double stars, he, no doubt, did
find a movement. He detected the remarkable fact that both the
apparent distance and the relative positions of the two bodies were
changing. But what was his surprise to observe that these
alterations were not of an annually periodic character. It became
evident then that in some cases one of the component stars was
actually revolving around the other, in an orbit which required many
years for its completion. Here was indeed a remarkable discovery. It
was clearly impossible to suppose that movements of this kind could
be mere apparent displacements, arising from the annual shift in our
point of view, in consequence of the revolution of the earth.
Herschel's discovery established the interesting fact that, in
certain of these double stars, or binary stars, as these particular
objects are more expressively designated, there is an actual orbital
revolution of a character similar to that which the earth performs
around the sun. Thus it was demonstrated that in these particular
double stars the nearness of the two components was not merely